It could be something she pecked, but most poisonous plants taste nasty to chickens so they don’t tend to eat them.
As yours were out free-ranging yesterday, green poop would not be unexpected. Any sudden and severe medical problem could have caused her to lose control of her back end and the green you saw was a natural emptying of what she had been eating, but in a messy uncontrolled way.
There are a few basics like checking the coop for parasites, worming (heavy worm loads can cause green poop and put a huge strain on a hen), checking everyone is breathing normally with no bubbling around the eyes or nostrils. Other than that, keep a close eye on the remaining 2 and if you see any signs of illness, get them to a vet, but with any luck this one sudden death may just be a one-off.